The selection of a metallic material for each application in the field of corrosion, as in any other field, dictates some compromise in material characteristics, such as cost, availability, fabricability, strength, ductility, hardness, magnetic properties, and degree of corrosion resistance provided by each material. No single material is best in all properties. A need has always remained for new metallic alloys that provide a better mix of properties for each corrosion situation at lower relative cost.
In some applications, it is also desirable that such alloys be substantially non-magnetic. One such application is for naval mine sweepers which most avoid destruction by magnetic mines. Non-magnetic alloys are also advantageous materials of construction for submarines since they allow the vessel to elude the magnetic anomaly detector systems that are employed to locate submerged submarines. These systems sense changes in the earth's magnetic field caused by metallic masses as large as steel submarines.
For various physical and metallurgical reasons there are many chemical elements that are not compatible in iron-base or nickel-base alloy systems. There are other elements, such as platinum and palladium that are quite compatible but too scarce and expensive to have been employed commercially in such alloys. Hence, most if not all of the practical commercial stainless steels, nickel-base corrosion-resistant alloys and related alloys are comprised of elements chosen from the same group. Nonetheless, significant advances have come from new combinations and proportions of these same elements. In addition to nickel, the most widely employed elements from this group are chromium, molybdenum, manganese, silicon and carbon. Also, of considerably wide usage are columbium (niobium), copper, titanium, and, almost by coincidence, cobalt and tantalum.
Other elements less widely selected for these corrosion-resistant alloys are tungsten, nitrogen, boron and rare earth elements, i.e., cerium, lanthanum, etc. Various combinations of these elements are used to formulate all iron-base or nickel-base corrosion resistant alloys of any significant employment. Hence, there has always remained in this field a continuing need for new combinations of these elements that will give better resistance in certain corrosive media or provide better combinations of such properties as fabricability, strength, toughness, or lower strategic element content and hence, lower cost.
For handling seawater and chloride-containing fluid streams, as in all other materials selections, a need has persisted for alloys of improved cost-effectiveness. For example, the element tantalum or the alloy of titanium containing 20% molybdenum and 0.2% palladium will effectively show almost no corrosive attack in boiling solutions of extremely aggressive oxidizing or reducing solutions, whether or not they contain chlorides. However, there isn't enough tantalum in the entire earth's crust to make it available for ordinary structural use. It is a semiprecious and extremely rare element almost like platinum. While titanium is far more plentiful, alloying it with 20 to 40% molybdenum and even a small amount of palladium results in an alloy of very high cost and unsuitability to production and fabrication by ordinary methods.
Also, there have been a few nickel-base alloys of very high total strategic element content that resist salt water and various corrosive chemical streams. Such alloys have often failed in media that do not readily attack titanium or tantalum alloys. Reducing strategic metal content has generally resulted in narrower performance capabilites.
Much of the alloy research in this field as in many other corrosion applications is directed toward materials that effectively meet narrower or more restricted service situations with both relatively low strategic element content and non-stringent production and fabrication requirements, and hence finished costs that are very much lower than those incurred with high strategic metal content alloys.
It is recognized that alloys intended for salt water service depend largely upon some combination of molybdenum and chromium contents. If they are to be castable by the usual lower cost production methods they will also contain some nickel and be of the austenitic, or face-centered-cubic crystalline structure. Variations in some other elements have been found to increase or decrease seawater resistance to some extent. Prior art alloys of this type have tended to contain about 20% Cr, 6% Mo and various Ni contents. I am applying for a U.S. patent on an improved alloy of about 18% Cr, 7.5% Mo plus nickel (Ser. No. 947,427 filed 12/29/86) and another alloy of about 24% Cr, 4.75% Mo plus nickel (Ser. No. 947,095 filed 12/29/86). In addition, my U.S. Pat. No. 3,947,266 covers alloys of about 26 to 30% Cr, 3 to 4% Mo plus nickel, that resist sea water and many other corrosive streams. The same is true for alloys of my U.S. Pat. No, 3,759,704 which contain 33 to 42% Cr, 3 to 8% Mo plus nickel.
Also, an alloy commercially available under the tradename of SANICRO 28, containing 27% Cr, 3.5% Mo plus Ni, was developed originally for production of phosphoric acid. It has now been found to be quite suitable for sea water service.
It is desirable to achieve adequate resistance with the minimum of Cr, Mo and, hence Ni. Since Mo is a much scarcer element than Cr, each weight percent reduction in Mo is several times more cost-effective than the equivalent weight percent reduction in Cr.
Fontana U.S. Pat. No. 2,214,128 describes an alloy containing from 1 to 4% manganese and 2-6% molybdenum. Fontana states that manganese, when used in the proportions set forth, adds the quality of ease of fabrication. Fontana actually presents only one example of his invention, which contains 3.40% Mo, 2.08% Mn, 1.98% Cu, 0.98% Si and 0.09% C.
Thyssen Rohrenwerkee Akiengesellschaft, British patent. No. 1,062,658, discloses the use of 3 to 12% manganese and 0.17 to 0.24% nitrogen in modified stainless steels containing 0 to 4% molybdenum and 0 to 0.15% columbium with no copper. Neither copper nor molybdenum are essential ingredients of that disclosure. Also, in the three examples disclosed in British patent No. 1,062,658, nickel contents are 10.1% or lower, and manganese contents are 6.7% or higher. It is further stated British patent No. in 1,062,658 that articles intended for exposure to seawater and/or to a sea atmosphere should contain 0 to 3% Mo, 0 to 0.15% Cb, 3 to 12% Mn, 15 to 22% Cr and 9 to 16% Ni. This reference further states that under conditions of local corrosion the preferred composition is up to 12% Mn, 1 to 4% Mo, 9 to 25% Ni, 17 to 25% Cr, 0.17 to 0.4% N, and up to 0.15% Cb.
My prior inventions, as described in Culling U.S. Pat. No. 4,135,919 and Culling U.S. Pat. No., 4,329,173, were disclosed for use in handling various concentrations of sulfuric acid.